This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A104255 #6 Oct 10 2018 10:31:08
%S A104255 0,0,2,13,94,866,9652,126689,1914412,32736453,624968662,13176422634,
%T A104255 304071291562,7623501667031,206342778454312,5996836998828457,
%U A104255 186254702081260312,6156752652130549218,215810382437839251562,7995774669321944270390,312215963278285442939062
%N A104255 a(n) = floor( (2n-1)!!/(2n) ).
%C A104255 Asymptotic estimate for A007769(n).
%H A104255 A. Khruzin, <a href="http://arXiv.org/abs/math.CO/0008209">Enumeration of chord diagrams</a>
%t A104255 Table[Floor[(2n-1)!!/(2n)],{n,20}] (* _Harvey P. Dale_, Nov 12 2012 *)
%K A104255 nonn
%O A104255 1,3
%A A104255 _Ralf Stephan_, Mar 02 2005